Conventional bearings, such as engine bearings or chassis bearings, in particular chassis bearings for the rear axle of a motor vehicle, comprise a rubber elastomer body which is supported in the chassis. The elastomer body has a central, inner bushing in the form of an inner bearing sleeve with a pin, which is guided therein and is pivotally connected to the wheel suspension, for example. In other bearings of similar construction, the pin is mounted on the chassis and the elastomer body is connected to the wheel suspension. The elastomer body is usually enclosed by a housing in the form of a one-part or multi-part outer bearing sleeve.
With conventional bearings of this type, vibrations of the wheel suspension or the engine are damped in relation to the chassis, the damping varying as a function of the characteristics of the elastomer body.
For comfortable travel in a vehicle, a softly matched chassis bearing or a soft engine bearing is desirable. A force acting on such a ready assembled chassis bearing is damped by soft adjustment up to a specific maximum vibration amplitude. At greater vibration amplitudes, such as those which occur in the braking or acceleration of a vehicle, especially when passing through resonance, the inner and outer bearing sleeves can initially abut. In this case, a hard bearing is desirable. In order to minimize the vibration amplitude on passage of the resonance, bearings with a high bearing stiffness are, in principle, used.
DE 40 36 538 A1 and DE 100 49 140 A1 have already disclosed chassis bearings, the stiffness of which can be controlled as a function of the driving conditions. DE 40 36 538 A1 discloses an assembly bearing which can also be used for adjusting the stiffness on the sleeve bearing of a chassis. The assembly bearing has two spring elements of which one spring element is permanently operative and the other spring element can be switched in by a control device as a function of the driving conditions. The switched-in spring element is activated by an actuating element. To increase the stiffness of the assembly bearing, a diaphragm can be subjected to a pressure medium through a pressure connection thereby moving a supporting part from its original position towards one of the two operative spring elements with which it comes into contact. This means that one of the two spring elements is no longer operative so that the stiffness of the assembly bearing is increased.
DE 100 49 140 A1 describes a chassis bearing, which can be adjusted to a greater stiffness according to the running conditions of the vehicle. The greater stiffness is here achieved by the activation of a second bearing. In the switched state, the first bearing is connected in parallel to the second bearing, so that together the spring constants add up to a greater aggregate stiffness.
A disadvantage of the adjustable bearings cited is that they have a very complex and costly construction. In each case two spring systems connected in parallel or in series are necessary.
United States patent application publication 2006/0220448 A1 describes a chassis bearing of the type described in the introductory part, in which the bearing stiffness can be increased in that the adjusting elements rest against the inner bearing sleeve and are fixed by pressure acting on the rear end faces of the adjusting elements. Here, the pressure required for fixing can only be fully built up once both adjusting elements are resting against their opposing stop faces. The inner bearing sleeve is fixed relative to the outer bearing sleeve solely by the pressure acting on the end faces of the adjusting elements and by a valve, which is closed for locking and prevents an exchange of pressure medium between the individual chambers. Disadvantageous is here that the pressure must be fully developed at the instant at which the inner bearing sleeve is to be fixed and that a compressible medium, such as air, cannot be used.